I wish I could just give you and answer but I can share some very recent data I have from my recent build. It is also a heavily insulated gravity feed, I had 12 sheets to use if needed so it is packed in fairly well. During the hottest part of the fire the firebox skin where the coals were was about 200 degrees f. The cook chamber was around 375. The outer skin was 53 degrees f. My insulation is rated about 1800 degrees and cost was about $16.00 a 2'x4' sheet 2" thick.